The 4-threonine analogues of oxytocin and of mesotocin and isotocin were prepared by solid-phase synthesis. [4-Threonine]-oxytocin is about twice as active as oxytocin in rat uterus assays in vitro and in vivo and about three times as active in fowl vasodepressor assays. It is slightly more active than oxytocin in rat or rabbit milk-ejection assays. When infused intravenously into water-loaded rats it causes much less depression of diuresis than does an equal dose of oxytocin. [4-Threonine]-oxytocin has much less vasopressor activity than oxytocin. [4-Threonine]-mesotocin also shows enhanced oxytocin-like properties. Its oxytocic activity is equal to or greater than that of oxytocin and its fowl vasodepressor potency is about the same as that of [4-threonine]-oxytocin, 1500 u./mg. It also has less antidiuretic and vasopressor activities than mesotocin. Thus 4-threonine analogues, containing nothing but common l-amino acids, appear to have more of the specific oxytocin-like properties and less of the vasopressin-like properties than do oxytocin or mesotocin. Thus they may be considered improvements on the natural hormones. In this respect they are unique among the reported synthetic analogues of natural peptide hormones.
Substitution of 4-threonine in the weakly-active analogue [3-leucine]-oxytocin also increases its oxytocic and fowl vasodepressor activities. Thus a threonine in the 4-position appears to endow oxytocin-like peptides with greater specific activities than do the amino acids that occur naturally in this position, glutamine and serine. These observations may be of interest when considered (a) from an evolutionary viewpoint, (b) in attempting to interpret relations between molecular structures and biological activities, and (c) as describing peptides with more of the desired properties of oxytocin and less of the undesired properties which might have therapeutic advantages over the natural hormone.